It is known to produce an image of an object by scanning a field of that object on a line-by-line basis. The resulting signal (the video signal) is displayed on a cathode ray tube and the video signal can be compared to a television video signal. The object may be scanned by a television camera. Where a microscopic specimen is to be examined, the t.v. camera is focussed on a field of the specimen by means of a microscope.
It is desirable to be able to select features of the displayed image and for this purpose it is of course possible to compare the video signal with a reference voltage. However because the scanning spot has a finite size the amplitude of the video signal takes a finite time to change at the beginning and end of each feature. The true beginning and end of each feature is when the amplitude of the video signal has changed by 50% of the total change which will take place (the half-amplitude points), and the amplitudes of these points varies with the amplitude of the signal change and the amplitude of the background level on which the features lie. Consequently, if the fixed voltage is used as the reference voltage, misleading results may result.
It is an object of this invention to provide an improved image analyser.
It is today common to provide an automatic analyzer for automatically analyzing the image of a cathode ray tube. Conventionally a specimen, for example, a blood sample, is brought into the field of vision of a microscope with a TV camera connected to the microscope so that effectively the TV camera is focused on a field of the specimen by means of that microscope. The output of the TV camera is connected to a cathode ray tube which thus displays a magnified image of the part of the sample within that field. Certain features will appear within that image and it is a requirement to electronically analyze that image. For example, one or more cells of any type may appear in the image and it may be desired to measure the area of that cell. An electron beam scans the screen of the microscope in a raster in a conventional manner. In order to measure the area of a cell (all taken as an example) it is necessary to analyze the input video signal so as to pick up the beginning and the end of the part of the video signal which represents that feature. The video signal does not change instantaneously because the scanning spot produced by the electron beam has a finite area, and the beginning and end of a feature is normally accepted to be when half the eventual change in amplitude of the video signal has occurred. In other words, there is a degradation of feature edges due to the finite scanning spot size of the TV camera. However, feature edge rise and fall times are also degraded due to the optical resolution limit of the microscope and the bandwith of the video amplifier which processes the signal on the TV camera. These latter two defects in resolution are normally more significant than the aforementioned finite scanning spot size defect condition. The reason for using the half amplitude points to define the feature edge is that this is the best approximation to the true size of the feature prior to undergoing the resolution restrictions of the optical and electronic measurement system. Also, it imposes an accurate measurement boundary for any feature, irrespective of the setting of the detection threshold.